RENEWABLE ENERGY SYSTEMS MONITORING
sited on rooftops or, more commonly now, adjacent to buildings. The most common problems are wind shading from surrounding buildings, lower-than-expected local wind speeds, or control gear failure. Local wind regimes are probably the most diffi cult renewable energy resource to predict, and because the output of a wind turbine is proportional to the cube of the wind speed (up to the rated output), this is probably the dominant factor in turbine performance.
This PV array at London South Bank University was left in the shade after a taller building was built next door
(COP) is maintained above 2.2 (depending on the electricity grid carbon factor used). The COP depends heavily on the temperature difference between the heat source (ground or outdoor air) and load (that is, the building heating circuit fl ow temperature). Other factors include the heat pump refrigerant charge, compressor motor effi ciency, cycling rates, system fl ow rates and heat exchanger cleanliness. With air source heat pumps, the external evaporator must be kept free of frost and ice. This is achieved by various means, all of which impose a parasitic heat requirement, and subsequent reduction in COP. In ground source systems, the ground temperature is rarely monitored, but this is a key factor in the system performance. If the ground temperatures do not recover between heating seasons, gradual degradation of performance may occur. Biomass boilers present an interesting
case. In the case of wood fuel, heat output is easier to monitor continuously than the fuel input, which can only be measured by deliveries. Where dual fuel systems are employed (such as a gas fi red boiler), it is necessary to measure the heat delivered by each boiler (which is required by the RHI). Controls can drift – or have unintended complex interactions – that can result in the low-carbon source being under-utilised. In order to maintain their optimum
effi ciency, biomass boilers are also likely to need more maintenance than gas-fi red boilers, and with the price of biomass fuels likely to rise, it is important to ensure systems operate at maximum performance. Small-scale wind turbines are often
42 CIBSE Journal August 2012
What to monitor Each technology discussed above will have different monitoring and reporting requirements, although each can ultimately be reported in terms of energy, money and carbon saved over a specifi ed period of time. If continuous monitoring is employed, there is an excellent opportunity to undertake condition monitoring, to check against expected savings, to report successes, and to ensure early rectifi cation of faults. Considering the high cost of renewable installations and the sensitivity of returns to real performance, not to do this would appear rather cavalier (see box on Calculating PV performance on previous pages). The FiTs scheme in the UK has seen
more than a gigawatt of PV installed in the UK. The RHI is promising to help renewable heat technologies become established by offering additional subsidised payments. Any widespread failure will discredit renewables and constitute a vast wasted investment. Proper monitoring, measurement and verifi cation is important for all energy systems, not least for renewables. By incorporating renewables into effective M&T systems, they can be properly viewed in conjunction with other carbon and energy reduction strategies – not as a separate side-show. If managed properly, these decentralised
and dispersed systems can benefi t the UK with widespread carbon reductions and less reliance on the imports of fossil fuels. And on a local level they offer a much better return of investment than the bank on the High Street. CJ
References 1. N. Moore and A.R. Day, Shading impact of a new building on an existing photovoltaic array in London, Sustainable Urban Development in Buildings and Environment, Chongqing, September 2011
PROFESSOR TONY DAY is energy service director at TEAM (EAA Ltd)
www.cibsejournal.com
Courtesy of
energyforlondon.org
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